Gravimeters are widely used in geological exploration to measure the first derivatives of the earth's gravitational field. Whilst some advances have been made in developing gravimeters which can measure the first derivatives of the earth's gravitational field because of the difficulty in distinguishing spatial variations of the field from temporal fluctuations of accelerations of a moving vehicle, these measurements can usually be made to sufficient precision for useful exploration only with land-based stationary instruments.
Gravity gradiometers (as distinct from gravimeters) are used to measure the second derivative of the gravitational field and use a sensor which is required to measure the differences between gravitational forces down to one part in 1012 of normal gravity.
Typically such devices have been used to attempt to locate deposits such as ore deposits including iron ore and geological structures bearing hydrocarbons.
International publication WO 90/07131 partly owned by the present applicants associated company discloses a gravity gradiometer. The gradiometer includes a gimbal bearing arrangement comprised of three concentric rings in which is mounted the sensing equipment. The sensing equipment generally comprises two spaced apart bars respectively located in shielded housings and each mounted on a web bearing. The instrument disclosed in that application is relatively complicated in that it includes a large number of parts and is relatively heavy which is a disadvantage particularly in airborne applications.
Gravity gradiometers of the type described above operate in a cryogenic environment and utilise superconducting circuits and materials. To ensure proper operation of the superconducting circuits, it is necessary to ensure that spurious magnetic fluxes do not produce currents in the superconducting circuits which result in erroneous measurements.